Waste Water Treatment Plants

How To Decrease Shutdown in Waste Water Treatment Plants

Background

A Waste Water Treatment Plant recently required a solution to the serious problem it was having with blockages in the system it used for the production of biogas. Blockages were occurring in its existing spiral heat exchangers which heated sludge prior to its anaerobic digestion in a digester.

The operator of the Plant was forced to carry out frequent and costly shutdowns to remove blockages in the existing heat exchanger.

The problem was analysed and a state-of-the-art solution was developed and adapted to the requirements of Plant.

Process

A large amount of organic waste was being produced and in order to reduce the volume of the waste and limit pollution, the residuals, or sludges, were sent to anaerobic digesters where biochemical degradation of the sewage sludge occurs and where bacteriological action takes place in an oxygen free, controlled temperature environment for further processing into biogas.

Anaerobic digestion of sewage sludge allows not only the reduction of discharge but also recovery of the energy produced in the form of biogas which has a high calorific value due to its rich methane composition.

The resultant biogas is stored in a gasometer and, after treatment to remove polluting or corrosive components, it can be used as a combustible to drive alternators and produce electrical power which, in turn, can be used either on site or discharged to the electricity grid.

The heating of sludge prior to the digester is a critical process.

The temperature of the sludge at the inlet of the anaerobic digester is a key feature in the process and has a significant influence on the retention time necessary for the digestion process to be completed by the bacteria.

To achieve appropriate results the temperature of the sludge must be controlled in a very narrow band in order to properly stabilize the sludge that goes to the digester. For example in 300C atmospheres, 10 days are required for mesophilic digestion, whereas at 200C, 25 days are required for digestion.

The heating of the sludge is carried out in a heat exchanger that must remain free of blockages and perform properly to achieve efficient digestion.

Product Specifications

The contents of sewage sludge can differ significantly from one treatment plant to another and the thermal properties of sludge, particularly its viscosity, can vary markedly.

Apart from viscosity, the thermal properties, density, specific heat and thermal conductivity of sludge are features that may affect the approach taken for the thermal design of the heat exchanger.

Once the thermal properties are assessed and agreed they are compared with fluid databases which contain the values for various sludges with different dry extract contents.

Benefits of rheological testing.

Rheology is the study of the flow of matter, primarily in a liquid state, but also as ‘soft solids’ or solids under conditions in which they respond with plastic flow rather than deforming elastically in response to an applied force.

Although it is not always needed, Fluid Dynamics can arrange rheological testing of a product sample to enable an analysis of the material’s thermal properties. Rheological testing provides a very accurate analysis which helps ensure the correct design of the heat exchanger.

Benefits of rheological testing.

Rheology is the study of the flow of matter, primarily in a liquid state, but also as ‘soft solids’ or solids under conditions in which they respond with plastic flow rather than deforming elastically in response to an applied force.

Although it is not always needed, Fluid Dynamics can arrange rheological testing of a product sample to enable an analysis of the material’s thermal properties. Rheological testing provides a very accurate analysis which helps ensure the correct design of the heat exchanger.

The risk of blockage is prevented with a corrugated tubular heat exchanger.

Design of the Tubular Heat Exchanger

The subject project in this case study used the CT-20 model Tubular Heat Exchanger the main features of which are:

Monotubular geometry

The CT-20 model consists of 2 concentric tubes in which the water used as the heating medium flows through the annular space while the sewage sludge flows through the inner tube.

Modular design

A design with flanged connections was selected to facilitate dismantling the units for ease of cleaning and to allow for the addition of more modules if increased performance should be required.

Process duty

Fluid Dynamics’ stainless steel tubular heat exchangers can process a wide range of duties with the option to run process by means of several lines in parallel for full capacity or, if one line requires cleaning or maintenance, that line can be removed from the line while production continues unabated.

Corrugation

The inner tubes of Fluid Dynamics’ stainless steel tubular heat exchangers are corrugated to provide maximum turbulence of the product reducing the inherent risk of fouling while increasing the thermal efficiency of the exchanger. In this case the tubes were HARD corrugated to give maximum turbulence to the product and provide the optimal heat exchange area.

Corrosion resistance

Due to the presence of phosphate and ammonium precipitated (struvite) in the case study, stainless steel tubular heat exchangers were designed using 316L stainless steel which provides much better protection against the risk of corrosion than 304 stainless steel does. Other materials such as duplex stainless steels are also available for applications where even higher resistance to corrosion is required.

Insulation

Insulation of tubular heat exchangers is optional and reduces the heat loss to the environment. In this case study the modules were individually insulated with mineral wool cladded with aluminium.

Case Study Key Figures

Process flowrate: 100 m³/h of sewage sludge

Dry extract content of the sludge: 6%

Temperature at the inlet of the digester: 38 ºC

Pressure drop of the sludge: 1 bar

Thermal duty: 600,000 kcal/h

Floor space: 800mm Wide x 12.5m Long x 2m High

Fluid Dynamics’ staff has extensive experience in all heat exchange applications including projects for waste water and sewage sludge processing and are happy to discuss any heat exchange issues.

Fluid Dynamics is Australian owned and operated. Established in 1981 it specialises in heat exchangers for all applications. It is an independent company and offers solutions based on the best heat exchanger for the duty required. ISO certified and also certified as a Recognised Defence Supplier, Fluid Dynamics offers quality and service to all industries for all applications.

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In over 35 years of operation, Fluid Dynamics has built an enviable reputation for being able to design, re-engineer, refurbish, clean and test a huge range of heat exchangers in its fully equipped facility. We carry a large range of stock and have the right equipment for even the most complex jobs.